1. Field of the Invention
The present invention relates to a transmission power control method and a communication system using the same, and more particularly to a transmission power control method in a CDMA (Code Division Multiple Access) system which performs multiple access using a spread spectrum technique in mobile communications, and a communication system using this method.
2. Description of Related Art
As is well-known, a CDMA system falls into two classes: a direct sequence (DS) system which spreads a conventionally modulated signal by using a high rate spreading code; and a frequency hopping (FH) system which resolves a symbol into elements called chips, and translates each chip into signals of different center frequencies at a high rate. Since the FH system is difficult to implement in the state of the art, the DS system is usually employed. Spread-spectrum radio systems differ from conventional communication systems for satellite data networks such as SCPC/FDMA (Single Channel Per Carrier/Frequency Division Multiple Access) systems, or TDMA (Time Division Multiple Access) systems in that the spread-spectrum radio systems transmit, at a transmitter side, a signal which is modulated by a common modulation, and then by a secondary modulation using a spreading code, which widens its signal bandwidth. At a receiver side, on the other hand, the wideband received signal is despread to restore the narrow band signal, followed by a conventional demodulation processing. The despreading is performed by detecting correlation between the spread-spectrum sequence of the received signal and a spreading code sequence which is generated at the receiving station, and peculiar to the channel. The capacity in terms of the number of subscribers in a cell is determined by an SIR (Signal-to-Interference Ratio) needed to achieve a required error rate because a CDMA system uses the same frequency band for the subscribers.
Applying the CDMA system to a mobile communication presents a problem in that received signal levels at a base station from respective mobile stations vary significantly depending on the locations of the mobile stations, and this arises a "near-far problem", in which a large power signal masks a small power signal, thereby reducing the number of mobile stations communicatable at the same time. In other words, a communication quality of a channel is degraded by signals of other communicators because the same frequency band is shared by a plurality of communicators and the signals from the other communicators become an interference.
FIG. 1 illustrates an interference state in a reverse (from mobile station to base station) channel due to other mobile stations. The reference characters BS1-BS3 designate base stations, and MS1-MS3 designate mobile stations in the cell associated with the base station BS1. When the mobile station MS1 closer to the base station BS1 than the mobile stat ion MS2 communicates with the base station BS1 at the same time with the mobile station MS2, the received power of the base station BS1 from the near mobile station MS1 will be greater than that from the faraway mobile station MS2. As a result, the communications between the faraway mobile station MS2 and the base station BS1 will be degraded owing to the interference from the near mobile station MS1.
To overcome this near-far problem, a transmission power control has been introduced. The transmission power control regulates received power at a receiving station, or the SIR determined by the received power, such that the received power or the SIR becomes constant regardless of the locations of mobile stations, thereby achieving uniform communication quality in a service area.
FIG. 2 shows a received power level at a base station when the transmission power control in a reverse direction is carried out, in comparison with a received power level when the power control is not carried out. Since a mobile station near the border to an adjacent cell receives interference from the adjacent cell, the degradation of communication quality due to the near-far problem occurs in both reverse and forward (from base station to mobile station) communications.
FIG. 3 illustrates an interference state of a forward channel from the base station BS1 to the mobile station MS3, due to the base stations BS2 and BS3 of other cells. As shown in this figure, signal powers of the other communicators become interference, and hence, transmission power control must be carried out to prevent the signal powers of the other communicators from growing much larger than the transmission power of the intended channel.
In particular, with regard to a reverse channel, each mobile station controls transmission power such that the received power thereof at the base station becomes constant. Since the interference is considered as white noise in the CDMA system, an error in the transmission power is the most important factor in determining the capacity in terms of the number of subscribers in a cell. For example, an error of 1 dB in the transmission power will reduce the capacity in terms of the number of the subscribers by about 30%.
On the other hand, with regard to a forward channel, since the signal of an intended channel and interferences caused by signals for other users within the cell propagate through the same path, they are subject to the same long interval fluctuations, the same short interval fluctuations, and the same instantaneous fluctuations, so that their SIR is kept constant. Therefore, the transmission power control is not necessary if the interference is caused only within a cell. Actually, however, interferences from other cells must be considered. This is because although the interference power from other cells undergoes instantaneous fluctuations due to Rayleigh fading as the interference power within the cell, its fluctuations differ from those of the intended signal.
FIG. 4 illustrates behavior of a received signal at a mobile station. In a CDMA system standardized by TIA of the United States, the transmission power control is not basically performed in a forward channel. Instead, a base station detects a frame error rate of a received signal, and increases the transmission power to a mobile station if the frame error rate exceeds a predetermined value. This is because a large increase in the transmission power will increase the interference to other cells. The transmission powers from base stations of other cells constitute an interference which fluctuates instantaneously.
FIG. 5 shows the operation principle of a first conventional closed loop transmission power control which is performed in accordance with a received SIR. In FIG. 5 (and FIG. 6), the reference character S designates the received power of a desired signal, I designates the received power of interferences, and pg designates a processing gain. The first conventional transmission power control in a CDMA system is performed such that an actual SIR agrees with a reference SIR which is determined in advance to provide a required communication quality. Here, the SIR is defined as the ratio of the received power of an intended signal to the interference power which is the sum total of thermal noise and interferences from users other than the intended user. In this first conventional method, an increase in the received signal power of the user to obtain the reference SIR will results in an increase in interference power to other users. This will form a vicious cycle which causes successive increases in transmission powers of respective mobile stations, and each of the mobile stations will come to transmit at its maximum transmission power.
FIG. 6 illustrates the operation principle of a second conventional closed loop transmission power control based on a received thermal noise level. The second transmission power control is performed in accordance with a ratio S/(I.sub.max +N), where S is the received signal level of an intended wave, I.sub.max is the maximum interference power caused by the maximum number of users that the system can accommodate, and N is the thermal noise power. In other words, the transmission power control is performed in accordance with the ratio of the level S to the level I.sub.max, which levels are measured from the thermal noise level N. In this case, even if the number of actual communicators within the cell is less than the maximum number, a mobile station will radiate such transmission power that ensures a required reception quality at the base station on the assumption that the maximum number of users are communicating at the same-time (SNR in FIG. 6 will be described later).
As a result, in either Case of FIGS. 5 and 6, a mobile station comes to radiate the maximum transmission power corresponding to the maximum capacity in terms of the number of users. This forces the mobile station to consume extra power. A similar problem will occur in a forward channel transmission from base station to mobile stations.